1. Field of the Invention
The present invention relates generally to an improved method and apparatus for detecting underground electrical conductors surrounded by less electrically conducting rock and more particularly to a method for detecting are veins or electrically conducting equipment located in underground tunnels or boreholes.
2. Description of the Prior Art
Several techniques are currently used in military operations to detect underground tunnels. These include visual observation of surface features, surface drilling, use of accoustical and seismic systems and the deployment of various surface and downhole electromagnetic (EM) wave propagation methods.
Of these techniques, the EM techniques are the most promising because they are less sensitive to munition firings and can be made less sensitive to random geologic structure noise. Two EM techniques presently in use are a cross-hole high frequency diffraction detection method claimed in U.S. Pat. No. 4,161,687, issued to Lytle, et al. and a cross-hole pulsed EM system (PEMSS-II) developed by the Southwest Research Institute and currently used by the U.S. and South Korean Armies.
R. J. Lytle, et al., in "Cross-borehole Electromagnetic Probing to Locate High Contrast Anomalies"; Geophysics, Vol. 44, pp. 1667-1676 (October 1979), discloses a theoretical basis for detecting tunnels by diffraction scanning.
All of these cross-hole techniques are designed to detect changes in the electrical parameters of the geologic medium caused by the tunnel intersection with a vertical plane between two drillholes. However, these techniques utilize downhole transmitting and receiving antennas that are connected to surface equipment by electrically conductive cable. This use of electrically conductive cable interferes with magnitude and phase shift measurements and prevents the reliable use of synchronous detection techniques.
In general, use of antennas and electromagnetic wave propagation methods in slightly conducting natural rock for remote sensing and mapping of subsurface geologic features, for applications in hardened military communications, and radio communications with miners working or trapped in underground tunnels has been reported in the literature. The subject area has been investigated for communications with submerged submarines. Review papers, Hansen, R. C., "Radiation and Reception with Buried and Submerged Antennas," IEEE Trans. on Ant. and Prop.; May 1963; and Moore, R. K., "Effects of Surrounding Conducting Medium on Antenna Analysis", IEEE Trans. on Ant. and Prop.; May 1963, trace the historical development of the canonical theory from its late 18th century beginning with Heaviside, O., "Electrical Papers", Vols. I and II, MacMillan and Company, Ltd., London, England 1882. The theoretical problem considered the interaction of antennas and EM field components with slightly conducting geologic medium. For radio communications, the problem considers radio wave propagation along the surface of the earth, direct paths through the earth, up over and down paths between submarines, and the possibility of a deeply buried natural waveguide in the earth. For geological investigations, the problem considers the detection of halos of chemically mineralized ore zones associated with faults and dikes, sandstone layers and voids in limestone that trap oil and gas, seams of coal, trona, potash, and anomalies that interfere with orderly extraction of valuable resources. Sommerfield, A., "Uber die Austreitung der Wallen in der Drathlosen Telegraphic", Ann. Physik, Ser 4 Vol. 81, No. 17, pp. 1135-1153, Dec. 1926, provided early theoretical insite into surface wave communications, and Wait, J. R. (guest editor) May 1963 issue of IEEE Trans. Ant. and Prop. vol. AP. 1, No. 3, contributed knowledge regarding communications and techniques for investigating subsurface geological features.
J. R. Wait and D. A. Hill, "Coaxial and Bifilar Modes on a Transmission Line in a Circular Tunnel", Preliminary Report to U.S. Bureau of Mines on Contract No. H0122061 (September 1974); relates to an investigation of propagation of guided waves in tunnels and formulated a theoretical model showing that monofilar and bifilar propagation modes exist for two-wire cable and trolley tracks and power cable types of conductors.
Also, a method for measuring the bulk electrical parameters of a region of the earth which involves measuring the intensity and phase shift values of the magnetic field of an electromagnetic wave simultaneously received in two boreholes is described in R. N. Grubb, P. L. Orswell and J. H. Taylor, "Borehole Measurements of Conductivity and Dielectric Constant in the 300 kHz to 25 MHz Frequency Range", Radio Science, Vol. II, No. 4 (April 1976).
J. R. Wait, "The Magnetic Dipole Antenna Immersed in a Conducting Medium", Proceeding of the IRE (Oct. 1952), points out that a fundamentally different power dissipation relationship exists between electric and magnetic dipole antennas. In the electric dipole case, the radial wave impedance near the dipole is largely real, whereas the impedance is imaginary in the case of the magnetic dipole. The large real impedance results in more energy dissipated near the electric dipole than flows out to large distances.
R. F. Harrington, "Time Harmonic Electromagnetic Fields", McGraw Hill, N.Y. (1961), describes a formula for calculating the current flow produced in a conductor by an incident electric field.
Synchronous detection principles are described by W. R. Bennett and J. R. Davey in "Data Transmission", McGraw Hill Book Company (1965).
Philip F. Panter, in "Modulation, Noise and Spectral Analysis Applied to Information Transmission", McGraw-Hill Book Co., pp. 461-503 (1965), describes the application of receiver frequency feedback compression techniques to decrease the occupied bandwidth of frequency or phase modulated radio signals.
Finally, U.S. Pat. No. 4,577,153, "Continuous Wave Medium Frequency Signal Transmission Survey Procedures for Imaging Structures in Coal Seams", by L. G. Stolarczyk describes a method for constructing images of structures in coal seams using the radio imaging method.